Honestly, I'm alright with the casting so far.
Garfield is the only one that I have no real opinion on yet, because he's a virtual unknown, but unknowns and superhero leads have a pretty good streak going for them. Same for Rhys-Ifan-Whatchamacallem as The Lizard. Emma Stone rocks, even if she'd visually be a better MJ, so that's cool. Martin Sheen is awesome casting for Uncle Ben, and Dennis Leary's casting strikes me as an Ultimate Captain Stacy, which I'm completely fine with since the original Captain Stacy always looked... well, 90.
The only thing I'm worried about is what they're gonna do with the costume. Either they're gonna change it drastically enough to bring home the idea that the movie deals with a brand new Spider-Man, or keep the previous series' suit, which was perfect, but would probably confuse everyone when posters and such would be released. Minor tweaks here and there while keeping the traditional design would be ideal, so hopefully that's what they'll do.